Optical mirror system with multi-axis rotational control

ABSTRACT

An optical mirror system with multi-axis rotational control is disclosed. The mirror system includes an optical surface assembly, and at least one leg assembly coupled to the optical surface assembly. The at least one leg assembly supports the optical surface above a substrate. A system and method in accordance with the present invention can operate with many different actuator mechanisms, including but not limited to, electrostatic, thermal, piezoelectric, and magnetic. An optical mirror system in accordance with the present invention accommodates large mirrors and rotation angles. Scanning mirrors can be made with this technique using standard surface-micromachining processes, or a deep RIE etch process. A device in accordance with the present invention meets the requirements for a directly scalable, high port count optical switch, utilizing a two mirror per optical I/O port configuration. An optical mirror in accordance with the present invention can be utilized in, but is not limited to, the following applications: optical add-drop multiplexers, wavelength routers, free-space optical interconnects, chip-level optical I/O, optical scanning displays, optical scanner (bar-codes, micro cameras), optical storage read/write heads, laser printers, medical replacement for glasses (incorporated with adaptive optics), medical diagnostic equipment, optical scanning for security applications.

This application is a continuation of application Ser. No. 09/879,025,filed Jun. 11, 2001, now U.S. Pat. No. 6,386,716.

FIELD OF THE INVENTION

The present invention relates generally to a MicroelectromechanicalSystem (MEMS) fabricated optical mirror system that is capable of beingtilted on two orthogonal axes, by means of electrostatically driven combdrives. Particular application to the use of these mirrors in thedeflection of optical space beams is emphasized.

BACKGROUND OF THE INVENTION

Fiber optic communication systems currently employ electro-opticswitching systems to route signals at central office switching centers.These electro-optic systems rely on converting the light output fromeach “incoming” fiber into electrical form, extracting the data contentin the resultant electrical signal, then utilizing conventionalelectrical switches to route the data content to a modulatable opticalsource that is coupled to a “destination” optical fiber. This detectionswitching remodulation process is expensive, complex, power consuming,and subject to component failure.

Alternate “All Optical” switching systems, employing mechanicallyactuated bulk optic and MEMS fabricated devices currently exist. Thesedevices utilize electromagnetic, piezoelectric and electrostaticactuators to physically move prisms, mirrors and portions of opticalfibers to affect switching of signals between optical fibers.

In addition fiber-to-fiber switches employing Grating Waveguides,Rowland Circle Gratings, and planar gratings, permit dedicated switchingbased on optical wavelength.

Cascaded binary tree configurations, employing switchable opticalcouplers using electrostatically variable index material, (LithiumNiobate and polymers), as well as Mach Zender interferometers utilizingthermoelectric heaters to affect unbalance, are also currently state ofthe art.

Many of the MEMS switches employ a space-beam deflection system similarto the electrical “Cross Bar” switch common in telephone system. Thisapproach requires that the number of mirrors for a given input/outputport count be determined by the square of the port count figure. Theoverwhelming number of mirrors dictated by this approach exceeds thatwhich can be produced with any realistic process yield, and survive anyreasonable operating period.

Except for some of the MEMS electrostatically actuated devices, none ofthe above methods of optical switching meets the requirements currentlybeing specified for high fiber port count, (up two 1024 by 1024) OpticalCross Connect switches. Problems of cost, reliability, insertion loss,polarization sensitivity, isolation, wavelength dependence, powerconsumption, and in some instances, switching speed, either individuallyor collectively mitigate against their use. Accordingly, what is neededis a system and method for overcoming the above-identified issues underthe constraint of a simple CMOS-compatible fabrication process.

An optical mirror system design is desired that has high-resolution 2-Dscanning capability and deflection capability, made with asurface-micromachining process. In order to achieve high-resolution,large mirror size and rotation angles are necessary.

The present invention addresses such a need.

SUMMARY OF THE INVENTION

An optical mirror system with multi-axis rotational control isdisclosed. The mirror system includes an optical surface assembly, andat least one leg assembly coupled to the optical surface assembly. Theat least one leg assembly supports the optical surface above asubstrate. A system and method in accordance with the present inventioncan operate with many different actuator mechanisms, including but notlimited to, electrostatic, thermal, piezoelectric, and magnetic. Anoptical mirror system in accordance with the present inventionaccommodates large mirrors and rotation angles. Scanning mirrors can bemade with this technique using standard surface-micromachiningprocesses, or a deep RIE etch process.

A device in accordance with the present invention meets the requirementsfor a directly scalable, high port count optical switch, utilizing a twomirror per optical I/O port configuration. An optical mirror inaccordance with the present invention can be utilized in, but is notlimited to, the following applications: optical add-drop multiplexers,wavelength routers, free-space optical interconnects, chip-level opticalI/O, optical scanning displays, optical scanner (bar-codes, microcameras), optical storage read/write heads, laser printers, medicalreplacement for glasses (incorporated with adaptive optics), medicaldiagnostic equipment, optical scanning for security applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical mirror system in accordance with thepresent invention.

FIG. 2 illustrates a side view of a preferred embodiment of the opticalsurface and support plate in accordance with the present invention.

FIG. 3 illustrates a top view of a vertical comb drive actuator inaccordance with the present invention.

FIG. 4 illustrates a side view of the vertical comb drive actuator ofFIG. 3.

DETAILED DESCRIPTION

The present invention relates to an optical mirror and more particularlyto an optical mirror system with a multi-axis rotational control. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention and is provided in the context ofa patent application and its requirements. Various modifications to thepreferred embodiment and the generic principles and features describedherein will be readily apparent to those skilled in the art. Thus, thepresent invention is not intended to be limited to the embodiment shownbut is to be accorded the widest scope consistent with the principlesand features described herein.

FIG. 1 illustrates an optical mirror system in accordance with thepresent invention. In a preferred embodiment, the optical mirror system100 in accordance with the present invention includes a plurality ofsupport legs 102 a-102 c coupled to a support plate 101, via a pluralityof connectors 108. The support plate 101 is coupled to an opticalsurface 103 through a via 109.

FIG. 2 illustrates a side view of a preferred embodiment of the opticalsurface 103 and support plate 101 in accordance with the presentinvention. The optical surface 103 is comprised of three laminatedlayers 103 a-103 c. Layer 103 a is the support layer which can be madeof polysilicon. The reflective layer 103 b (typically a thin metal) hasthermal coefficient (TCE) much greater than the layer 103 a, causingunwanted curvature of the optical surface in response to temperaturevariation. A third layer 103 c can be added “on top” or “beneath” thereflective layer 103 b having TCE lower than both the other layers. TheTCE/thickness of this third layer 103 c is selected to control thetemperature-induced curvature.

Preferably oxide is utilized for the third layer 103 c. However,materials other than “oxide” could be used for the third layer 103,provided their TCEs match the needed parameters.

The mirror substrate is connected to the support plate 101 through a via109. The support plate 101 provides a mechanical attachment point forthe actuators, isolating the optical surface 103 from the distortingmicromechanical forces of the actuators. Supporting the optical surface103 with a single, central connection results in symmetric mechanicalboundary conditions. Any shallow curvature induced in the opticalsurface 103 by thermal or intrinsic stresses will result in a parabolicdeformation of the optical surface 103, which can be corrected withspherical optics. In addition, the support plate 101 provides extrasurface area for increased heat dissipation from the optical surface103, resulting in greater optical power handling capability.

Each of the connectors 108 a-108 c (FIG. 1) are coupled to an actuator105. FIG. 3 illustrates a top view of a vertical comb drive actuator inaccordance with the present invention. FIG. 4 illustrates a side view ofthe vertical comb drive actuator of FIG. 3. In this embodiment, thereare three portions to the actuator system 105, first end portion 302,second end portion 304 and a middle portion 306. The end portions 302and 304 engage the middle portion 306 through interdigitated teeth 308and 310. In this embodiment, each of the end portions 302 and 304comprise three electrically isolated actuators 302 a-302 c and 304 a-304c, respectively.

Accordingly, if actuators 302 a and 304 a are activated, the system 105pulls “up”. If actuators 302 b and 304 b are activated the system 105holds the current position and if actuators 302 c and 304 c areactivated the system 105 pulls down. Accordingly, the actuator system105 can move the mirror in various ways dependent upon the voltagesapplied to the motors 302 a-302 c and 304 a-304 c. Although thisactuator system 105 has been described in the context of a threeposition (up, down and hold position) system, one of ordinary skill inthe art readily recognizes that a two position (up or down), (holdposition or down), (hold position or up) could be provided by using twoactuators rather than the three actuators in the system disclosedherein.

Referring back to FIG. 1, each of the legs 108 is coupled to a substrateby an anchor 107. Although a plurality of legs are shown, one ofordinary skill in the act recognizes there may be as few as one leg andthat use would be within the spirit and scope of the present invention.

The optical surface 103 is suspended on a plurality of support legs 102that lift it above the surface of a chip. The support legs 102 cause themirror to tilt through a large angle. The tilt-angle is greater than theangle that could be obtained using a standard sacrificial layer(typically 1-3 microns) as separation between the mirror and asubstrate.

In a preferred embodiment, the support legs 102 connect tangentially tothe side of the mirror. The support legs 102 can be rectangular or inthe shape of an arc along the edge of the mirror in the case of a roundor elliptical mirror. As the support legs tilt up, the mirror rotatesslightly to relieve stress due to small lateral movement in the supportlegs.

The support legs 102 can be either rigid or flexible. Flexible legs canbe used as springs, for the case in which a parallel-plate actuatorapplies force to the support plate beneath the mirror. By distributingthe bending over the length of a flexible support leg, the maximum shearand tensile stresses in the device are reduced, compared to a rigidsupport leg that concentrates the bending at flexures. Flexibleactuators 105 can be driven thermally (preferably by a resistive heateron each support leg) to cause the mirror to tilt.

Rigid support legs can be connected to the actuators 105 near thesurface of the chip. As the support leg tilts, powered by an actuator,the attached edge of the mirror can be raised or lowered. An actuatorcan be incorporated that facilitates differential capacitance sensing.

A system and method in accordance with the present invention can operatewith many different actuator mechanisms, including electrostatic,thermal, piezoelectric, magnetic, etc. Among electrostatic actuators, itsupports parallel-plate actuation between the mirror or beneath theelectrodes. The actuator can act on the support plate beneath themirror, or it can act on the support legs. In one implementation, byadjusting the coupling of the actuator to the support legs, the maximumrotation angle of the mirror can be traded off against the maximumapplied voltage.

An optical mirror system in accordance with the present inventionaccommodates large mirrors and rotation angles. Scanning mirrors can bemade with this technique using any micromachining processes. All-flexuredesigns of bi-axial scanning mirrors have superior device density,reliability, and repeatability characteristics. The system has thefollowing advantages over conventional optical mirror systemarchitectures.

1. Arbitrary equilibrium: The lengths of the support arms can be changedto adjust the equilibrium position of the mirror. The actuator does notnecessarily act directly on the mirror surface (as it would in aparallel-plate design). The equilibrium angle can be changed without asignificant change in the performance of the device.

2. Custom processing not necessary: This approach uses standardmicromachining processes. Since the mirror is lifted away from thesubstrate by the support legs, insulation on the frame is not necessary.The only insulator needed is at the substrate. If an insulator in thesupport legs is possible, then a wider variety of design options arealso available.

3. Nested frames and bimorphs dedicated for self-assembly are notnecessary: If bimorphs are needed, they can be incorporated into thesupport legs or elsewhere. By removing nested frames and extraneousbimorphs used in self-assembly, the size of the device is reduced,allowing more space for actuators or a higher density of devices.

4. Pin-and-staple hinges not needed: The support mechanism can besuspended on an all-flexure mounting. The tilt angle of these supportsis relatively small (within the shear and torsion limits of a properlydesigned support leg or hinge), and therefore pin-and-staple hinges arenot needed.

5. Mirror curvature symmetric and small: The mirror can be mechanicallydecoupled from the undesired deformation of the flexure hinges. Byconnecting the mirror to the support plate with a single via at itscenter, the mirror flatness is not affected by forces that develop inits suspension. In addition, if there is a stress gradient in themirror, the symmetric boundary conditions will typically result in aparabolic shape, which can be readily integrated into an optical systemusing off-the-shelf spherical optics. Non-spherical deformations of themirror, typically the result of asymmetric boundary conditions, cancause deformations in the mirror that can cause optical loss through aswitch. Electrodes directly beneath the mirror need not be used,removing creases that can occur in the optical surface because ofconformal deposition over the electrodes.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. For example, although it is disclosed in the preferredembodiment that the mirror rotates in a first and a second direction,the mirror can rotate in a plurality of directions (i.e. twistingmotion) dependent upon the electrostatic forces applied thereto.Accordingly, many modifications may be made by one of ordinary skill inthe art without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. An optical mirror system for directing a beam oflight, comprising: an optical surface assembly; at least one support legcoupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated whereinthe optical surface assembly comprises a reflective layer coupled to asupport plate through a via.
 2. The optical mirror system of claim 1wherein the reflective layer comprises a layer of metal.
 3. The opticalmirror system of claim 1 wherein the at least one actuator mechanismcomprises an actuator selected from the group consisting of anelectrostatic drive, a parallel-plate electrostatic drive, anelectrostatically driven comb drive, a vertical comb drive, aninterdigitated electrostatic actuator, a rotatable electrostatic drive,a torsional electrostatic drive, a rotatable interdigitatedelectrostatic drive, a bi-directional actuator, a thermal actuator, amagnetic actuator, and a piezoelectric actuator.
 4. The optical mirrorsystem of claim 1 wherein the optical mirror system comprises an opticalscanner.
 5. An optical mirror system for directing a beam of light,comprising: an optical surface assembly; at least one support legcoupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated whereinthe at least one support leg is connected tangentially to an edge of theoptical surface assembly.
 6. An optical mirror system for directing abeam of light, comprising: an optical surface assembly; at least onesupport leg coupled to the optical surface assembly; and at least oneactuator mechanism coupled to the at least one support leg; wherein theoptical surface assembly is positioned above a substrate by the at leastone support leg when the at least one actuator mechanism is actuatedwherein the at least one support leg is connected radially to an edge ofthe optical surface assembly.
 7. An optical mirror system for directinga beam of light, comprising: an optical surface assembly; at least onesupport leg coupled to the optical surface assembly; and at least oneactuator mechanism coupled to the at least one support leg; wherein theoptical surface assembly is positioned above a substrate by the at leastone support leg when the at least one actuator mechanism is actuatedwherein the at least one support leg is flexible.
 8. An optical mirrorsystem for directing a beam of light, comprising: an optical surfaceassembly; at least one support leg coupled to the optical surfaceassembly; and at least one actuator mechanism coupled to the at leastone support leg; wherein the optical surface assembly is positionedabove a substrate by the at least one support leg when the at least oneactuator mechanism is actuated wherein the at least one support leg isnominally rigid.
 9. An optical mirror system for directing a beam oflight, comprising: an optical surface assembly; at least one support legcoupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated whereinthe at least one support leg when the at least one actuator mechanism isactuated wherein the at least one support leg is coupled to thesubstrate with at least one anchor attached to the substrate and atleast one flexible beam connected between the at least one anchor andthe at least one support leg.
 10. An optical mirror system for directinga beam of light, comprising: an optical surface assembly; at least onesupport leg coupled to the optical surface assembly; and at least oneactuator mechanism coupled to the at least one support leg; wherein theoptical surface assembly is positioned above a substrate by the at leastone support leg when the at least one actuator mechanism is actuated,wherein the at least one actuator mechanism comprises an actuatorselected from the group consisting of an electrostatic drive, aparallel-plate electrostatic drive, an electrostatically driven combdrive, a vertical comb drive, an interdigitated electrostatic actuator,a rotatable electrostatic drive, a torsional electrostatic drive, arotatable interdigitated electrostatic drive, a bi-directional actuator,a thermal actuator, a magnetic actuator, and a piezoelectric actuatorand wherein the thermal actuator comprises a resistive heater on the atleast one support leg.
 11. An optical mirror system for directing a beamof light, comprising: an optical surface assembly; at least one supportleg coupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated whereinthe at least one support leg is rotated to position an attached edge ofthe optical surface assembly above the substrate when at least oneactuator mechanism is actuated.
 12. An optical mirror system fordirecting a beam of light, comprising: an optical surface assembly; atleast one support leg coupled to the optical surface assembly; and atleast one actuator mechanism coupled to the at least one support leg;wherein the optical surface assembly is positioned above a substrate bythe at least one support leg when the at least one actuator mechanism isactuated wherein the at least one support leg is rotated to position anattached edge of the optical surface assembly above the substrate whenat least one actuator mechanism raises a portion of the at least onesupport leg.
 13. An optical mirror system for directing a beam of light,comprising: an optical surface assembly; at least one support legcoupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated,wherein the at least one support leg is rotated to position an attachededge of the optical surface assembly above the substrate when the atleast one actuator mechanism raises a portion of the support leg andanother actuator mechanism lowers a portion of the support leg.
 14. Anoptical mirror system for directing a beam of light, comprising: anoptical surface assembly; at least one support leg coupled to theoptical surface assembly; and at least one actuator mechanism coupled tothe at least one support leg; wherein the optical surface assembly ispositioned above a substrate by the at least one support leg when the atleast one actuator mechanism is actuated wherein the at least oneactuator mechanism comprises a bi-directional actuator, wherein aportion of the at least one support leg is raised when thebi-directional actuator is pulled away from the substrate and theportion of the at least one support leg is lowered when thebi-directional actuator is pulled towards the substrate.
 15. An opticalmirror system for directing a beam of light, comprising: an opticalsurface assembly; at least one support leg coupled to the opticalsurface assembly; and at least one actuator mechanism coupled to the atleast one support leg; wherein the optical surface assembly ispositioned above a substrate by the at least one support leg when the atleast one actuator mechanism is actuated wherein the actuator mechanismcomprises at least two bi-directional actuators, wherein the at leastone support leg is rotated when at least one actuator is pulled awayfrom the substrate while another actuator is pulled toward thesubstrate.
 16. An optical mirror system for directing a beam of light,comprising: an optical surface assembly; at least one support legcoupled to the optical surface assembly; and at least one actuatormechanism coupled to the at least one support leg; wherein the opticalsurface assembly is positioned above a substrate by the at least onesupport leg when the at least one actuator mechanism is actuated whereinthe at least one actuator mechanism and the at least one support legcomprise a layer of single crystal silicon.
 17. An optical mirror systemfor directing a beam of light, comprising: an optical surface assembly;at least one support leg coupled to the optical surface assembly; and atleast one actuator mechanism coupled to the at least one support leg;wherein the optical surface assembly is positioned above a substrate bythe at least one support leg when the at least one actuator mechanism isactuated wherein the at least one actuator mechanism and the at leastone support leg comprise a layer of polycrystalline silicon.
 18. Anoptical mirror system for directing a beam of light, comprising: anoptical surface assembly; at least one support leg coupled to theoptical surface assembly; and at least one actuator mechanism coupled tothe at least one support leg; wherein the optical surface assembly ispositioned above a substrate by the at least one support leg when the atleast one actuator mechanism is actuated wherein at least one actuatormechanism further comprises: means for sensing to determine the positionof the actuator mechanism.
 19. An optical mirror system for directing abeam of light, comprising: an optical surface assembly; at least onesupport leg coupled to the optical surface assembly; and at least oneactuator mechanism coupled to the at least one support leg; wherein theoptical surface assembly is positioned above a substrate by the at leastone support leg when the at least one actuator mechanism is actuatedwherein the optical mirror system comprises an optical scanner.
 20. Anoptical mirror system for directing a beam of light, comprising: anoptical surface assembly coupled to a substrate; and at least oneactuator mechanism coupled to the optical surface assembly, wherein theoptical surface assembly is positioned when the at least one actuatormechanism is actuated, wherein the optical surface assembly comprises areflective layer coupled to a support plate through a via.
 21. Theoptical mirror system of claim 20 wherein the reflective layer comprisesa layer of metal.
 22. The optical mirror system of claim 20 wherein theat least one actuator mechanism comprises an actuator selected from thegroup consisting of an electrostatic drive, a parallel-plateelectrostatic drive, an electrostatically driven comb drive, a verticalcomb drive, an interdigitated electrostatic actuator, a rotatableelectrostatic drive, a torsional electrostatic drive, a rotatableinterdigitated electrostatic drive, a bi-directional actuator, a thermalactuator, a magnetic actuator, and a piezoelectric actuator.
 23. Theoptical mirror system of claim 20 wherein the optical mirror systemcomprises an optical scanner.
 24. An optical mirror system for directinga beam of light, comprising: an optical surface assembly coupled to asubstrate; and at least one actuator mechanism coupled to the opticalsurface assembly, wherein the optical surface assembly is positionedwhen the at least one actuator mechanism is actuated, wherein the atleast one actuator mechanism further comprises: means for sensing todetermine the position of the actuator mechanism.
 25. An optical mirrorsystem for directing a beam of light, comprising: an optical surfaceassembly coupled to a substrate; at least one support leg coupling theoptical surface assembly to the substrate; and at least one actuatormechanism coupled to the optical surface assembly, wherein the opticalsurface assembly is positioned when the at least one actuator mechanismis actuated wherein the at least one support leg is connectedtangentially to an edge of the optical surface assembly.
 26. An opticalmirror system for directing a beam of light, comprising: an opticalsurface assembly coupled to a substrate; at least one support legcoupling the optical surface assembly to the substrate; and at least oneactuator mechanism coupled to the optical surface assembly, wherein theoptical surface assembly is positioned when the at least one actuatormechanism is actuated wherein the at least one support leg is connectedradially to an edge of the optical surface assembly.
 27. An opticalmirror system for directing a beam of light, comprising: an opticalsurface assembly coupled to a substrate; at least one support legcoupling the optical surface assembly to the substrate; and at least oneactuator mechanism coupled to the optical surface assembly, wherein theoptical surface assembly is positioned when the at least one actuatormechanism is actuated wherein the at least one support leg is flexible.28. An optical mirror system for directing a beam of light, comprising:an optical surface assembly coupled to a substrate; at least one supportleg coupling the optical surface assembly to the substrate; and at leastone actuator mechanism coupled to the optical surface assembly, whereinthe optical surface assembly is positioned when the at least oneactuator mechanism is actuated wherein the at least one support leg iscoupled to the substrate with at least one anchor attached to thesubstrate and at least one flexible beam connected between the at leastone anchor and the at least one support leg.
 29. An optical mirrorsystem for directing a beam of light, comprising: an optical surfaceassembly coupled to a substrate; at least one support leg coupling theoptical surface assembly to the substrate; and at least one actuatormechanism coupled to the optical surface assembly, wherein the opticalsurface assembly is positioned when the at least one actuator mechanismis actuated wherein the at least one actuator mechanism and the at leastone support leg comprise a layer of single crystal silicon.
 30. Anoptical mirror system for directing a beam of light, comprising: anoptical surface assembly coupled to a substrate; at least one supportleg coupling the optical surface assembly to the substrate; and at leastone actuator mechanism coupled to the optical surface assembly, whereinthe optical surface assembly is positioned when the at least oneactuator mechanism is actuated wherein the at least one actuatormechanism and the at least one support leg comprise a layer ofpolycrystalline silicon.
 31. An optical mirror system for directing abeam of light, comprising: an optical surface assembly coupled to asubstrate; at least one support leg coupling the optical surfaceassembly to the substrate; and at least one actuator mechanism coupledto the optical surface assembly, wherein the optical surface assembly ispositioned when the at least one actuator mechanism is actuated whereinthe optical mirror system comprises an optical scanner.